Nonlinear stress relaxation behavior of ductile polymer glasses from large extension and compression

Abstract

We study nonlinear stress relaxation behavior of several polymer glasses during ductile large deformation of either extension or compression over a wide range of deformation rate at room temperature. In the pre-yield regime, the stress decline is logarithmically slow and essentially independent of the applied rate. The stress relaxation from the post-yield regime shows universal time rescaling, i.e., its time dependence is identical after multiplying the elapsing time with the deformational rate, for different polymer glasses undergoing either uniaxial extension or compression. After initial fast “plastic” relaxation, the stress decay returns to the same manner as that displayed by the pre-yield stress relaxation. After reviewing available modeling, we propose to interpret these results in terms of a recent picture for polymer glasses under large deformation that envision a chain networked embedded in a glassy matrix. Specifically we suggest that a) the enhanced molecular mobility during deformation is produced by load-bearing strands (LBSs) of the chain network, b) the initial high molecular mobility upon termination of external deformation survives thanks to the residual chain tension in LBSs, and c) the initial rapid “plastic relaxation” ceases when chain tension drops below a threshold that can no longer keep the segments (surrounding LBSs) activated.